An efficient electrically-pumped light emitter integrated in the standard CMOS technology has been so far the Holy Grail of the monolithic electronic-photonic integration. As a matter of fact, rapid advances in Si photonics over the last decade have enabled mass production of higher-functionality and lower-cost photonic integrated circuits, in which all the active and passive components such as waveguides, couplers, modulators, photodetectors, etc., except for the light source, can be fabricated side by side with both digital and analog circuitry in a silicon CMOS foundry. The obvious main advantages of a fully integrated light source are cost reduction, yield, easing of the packaging, link-budget improvement and a consequent power consumption reduction.
Among the different pathways leading to the on-chip integration of the light source, epitaxial lasers on silicon comprising active regions based on III-V or SiGe heterostructures have attracted a wide interest. In particular, Ge heteroepitaxial layers on Si are very promising since key photonic components for this material system, including high speed detectors and modulators, have been successfully integrated in standard CMOS process flow. Thus, Ge is now a “fab”-compatible material produced by means of fully qualified production processes and is considered one of the most promising materials for “more than Moore” device development.
An optically pumped Ge-on-Si laser demonstrating continuous-wave (cw) operation at room temperature has already been fabricated, as reported by J. Liu et al. Optics Letters 35, 679 (2010). In this approach, although Ge has an indirect band gap, the authors exploit a tensile strain in the Ge layer caused by a difference in thermal expansion coefficients between the Ge layer itself and the silicon substrate and accumulated during the fabrication process. Strain ε is defined as
      ɛ    =                  a        -                  a          0                            a        0              ,wherein a denotes the lattice constant of the strained lattice and a0 denotes the lattice constant of the unstrained lattice of the solid state material under consideration. Moderate strain of the order of 0.2% was reported sufficient to reduce the energy difference between the  and L valleys in the conduction band of the energy band structure of Germanium. Free electrons, incorporated through n-type doping, can fill up the low-lying L valley so that injected electrons do not thermalize at L but at , being thus available for radiative recombination through a direct transition.
The net gain is determined by the competition between this optical gain enhancement and the optical loss from additional free carrier absorption. In optically pumped bulk-Ge lasers, using a very high level of doping of 8×1019 cm−3, a net optical gain as high as 500 cm−1 could be achieved, as reported by X. Sun et al. IEEE J. of Sel. Topics in Quantum Electr. 16, 124 (2010).
The presence of a thermal tensile strain of 0.2% can double such a value. However the maximum reported gain for highly doped, thermal tensile strained structures is 50 cm−1, owing to the difficulties in to real high- and ultra-high n-doping in Ge due to donor solubility, dopant activation, and material processing. Therefore P. H. Lim et al., Optics Express 17, 16358 (2009), proposed to reduce the need of high-doping levels by externally increasing the tensile strain in the Ge epi-layer using micromechanical engineering.